1. Field of the Invention
This invention relates to a sheet transfer method for transferring piled sheets in units of a predetermined number, and a system thereof.
2. Description of the Related Art
There has been widely known a method in which piled sheets are deposited on a belt conveyor or a roller transfer unit for transferring purposes. However, such a conventional technique has a drawback in that the edges of the pile are tilted during the transfer. This problem is hereinafter referred to as unalignment. In order to obviate such unalignment, it is necessary to effect the realignment of the edges of the tilted pile at a subsequent step, and to have the realigned pile subjected to a process such as packaging, drilling, cutting or the like. As a much simpler and general method, sheets are transferred by hands.
In some applications where products include high quality surface decorative sheets, whose surfaces have undergone a special processing or coating, and presensitized printing plates to which a photosensitive resin has been applied, there has been a longstanding demand for a transfer system which is capable of transferring a pile from one process to the next process during the manufacture thereof with the edges of the products aligned.
During the transfer of piled sheets to a subsequent process, there has been a common practice, in order to prevent the unalignment of the pile, to attach tapes to a plurality of locations along the edge of the pile or to clamp the pile with a clamper. However, in view of the quality and appearance, the attachment of tapes is restricted to certain applications. Meanwhile, clamping the pile may cause the products to be deformed or damaged, and has a drawback in the time required for the mounting and dismounting of the clamps.
Alternatively, there may be considered other transfer techniques such as the dragging of a pile with a clamper or the deposition of a pile on a movable air table by introducing the table into a stack of sheets. These alternatives, however, involve the drawing of a pile which in turn may lead to the surface of a product being damaged.
In the field of products interested in this application, a protective sheet, called an interleaf, is usually inserted between sheets in order to protect the surface of the sheet from damage. During the transfer, however, the lowermost sheet of a pile is transferred without such an interleaf, so that the lowermost sheet is directly brought in contact with transfer rollers or an air table. This renders the quality of the lowermost sheet least ensured, and hence operators manually carry such products. With great care, the operators manually transfer the products so as not to disturb the integrity of each pile of sheets. As such, this manual operation results in muscular fatigue, known as CTD, or cumulative trauma disorder.
In addition to the above-mentioned alternatives, there is proposed a transfer system wherein operations are automated by robots, but this system requires a great amount of investment.
Examples of existing transfer techniques include U.S. Pat. No. 3,209,931, issued to Hawkes, wherein piles of flexible sheets are unstacked, trimmed and restacked by the use of a straddling type transfer table. This patent, however, discloses no teaching about the actuation of a transfer unit associated with the actuation of the loading and unloading subsections housed therein, substantially at the same speed in directions opposite to each other, to destack and restack a pile to be transferred without kinks in the pile or scratches on the topmost sheet of the remaining stack. Moreover, this method is disadvantageous in that the front end of the table is forcibly wedged into a stack overcoming friction, which causes the pile to be damaged or deformed.
In addition to the above, U.S. Pat. No. 4,055,258, issued to Schneider, discloses a sheet material destacking machine. This machine includes a hydraulically-driven gripping device which clamps a partial stack, preliminarily segregated by a separating roller, to complete the separation of the partial stack from the remaining stack. This machine inevitably suffers, when it is employed in the field of products interested in the present application, from the same drawbacks as already mentioned above, that is, the deformation or damaging of the products by the clamp.